Recently, as communication networks have speeded up, plural applications have come to coexist to meet requests from users, and high quality communication has been demanded. Accordingly, priority control is needed for each of such applications.
In particular, to ensure communication quality is mandatory for real-time applications for voice communications and video phones.
A technique for ensuring reliability of communication quality as described above will be TCP (transmission control protocol).
TCP is a protocol capable of performing secure data transfer through a logical connection independent from end-end media. Besides, TCP is responsible for part of congestion control in networks.
Specifically, for example, an acknowledgement packet is transmitted by TCP to acknowledge receipt of a transmitted segment. If the acknowledgement packet has not been received, the segment is considered as having been lost and retransmission control is then executed.
Further in this case, a transmission rate is decreased by decreasing a parameter called a congestion window. After once decreasing the transmission rate, the TCP works to gradually increase the rate. There are two methods of increasing this rate, e.g., “slow start” and “congestion avoidance”.
Since quality is more influenced by the “slow start” among the two methods, to avoid the “slow start” as much as possible is important for TCP control.
On the other side, as communication control dependents on media, there is link-by-link control which is resource access control for media. An example thereof will be CSMA/CA (carrier sense multiple access with collision avoidance) according to a random access scheme (wireless LAN (local area network)) for shared media.
Generally in communication using an end-end protocol such as TCP, plural communications (flows) are usually present on one link-by-link protocol such as CSMA/CA.
Further, a common way of thinking protocol layers suggests that end-end control and link-by-link control are performed independently.
A MAC (media access control) protocol for wireless LAN systems, which is defined by IEEE (Institute of Electrical and Electronics Engineers) 802.11, belongs to the link-by-link control.
In a wireless LAN, a wireless LAN terminal device transmits a data frame. Upon elapse of a time called SIFS (short inter frame space) since a receiving terminal device which receives the data frame completes receipt thereof, the receiving terminal device transmits an acknowledgement frame (which is called an ACK frame) as an acknowledgement frame in the wireless LAN.
In this case, a MAC address of the wireless LAN terminal device as a transmission terminal of the data frame is designated as a destination address of the ACK frame.
The SIFS time is the shortest possible acknowledgment time in consideration of propagation time, etc. The SIFS time varies depending on the modulation method of the physical layer. According to IEEE802.11b and 11g, the SIFS time is ten seconds.
If the wireless LAN terminal device has not yet received the acknowledgement frame (ACK frame) at predetermined timing even after elapse of the SIFS time, the wireless LAN terminal device retransmits the data frame.
Also, when no ACK frame is received in response to the retransmitted data frame, the wireless LAN terminal device carries out retransmission repeatedly up to the maximum number of retransmissions.
The maximum number of retransmissions is seven as a default value which depends on actual implements of the wireless LAN terminal device. If the manufacturer or vendor changes, the maximum number of retransmissions changes.